Effect of the Vis-term

The temperature dependence of the viscosity of the liquid and thereby the boundary layer thickness upon cooling and heating is taken into consideration with the viscosity term Vis = Following the suggestion of Sieder and Tate [505], that experimental data for heat transfer in pipes upon heating and cooling correlated upon inclusion of Vis , this expression was also accepted in most research studies over heat transfer in mixing. 

In Studies, which separately investigated the effect of viscosity, it emerged that the exponent of Vis is not constant, but can depend upon the stirrer type, the nature of the heat transfer surface and the nature of the test liquid. 

Thus Dunlap and Rushton [105] found in heat transfer experiments in bunched vertical pipes, that the differences between cooling and heating had to be taken consideration with Vis  

Hruby [222J found in cooling experiments with a Newtonian oil in the range 0.32 Vis 320, that the numerical value of the exponent m in the Vis term depended upon the numerical value of Vis  

Hackl [176J found in the range 80 Vis 900 a slightly lower value w=-0.265 Vis- (7.28) 

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The question of the effect of the Vis term was the main reason for Zlokarnik s study [614], Heat transfer was investigated in a stirred tank with anchor stirrer with D/d = 1.02 upon cooling and heating, whereby 8 liquids were utilized, whose viscosities lay between 1 and 10 mPa s and whose temperature coefficients of the viscosity yg, exp. (7.6), varied widely ... 

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